Tat is an unusual transcriptional activator that operates by binding to an RNA site (TAR) and enhancing elongation from the viral LTR. Tat and TAR form a larger complex with the cellular elongation factor P-TEFb, composed of cyclin T1 (CycT1) and Cdk9 subunits. Tat also interacts with other proteins of the transcriptional machinery. P-TEFb exists in two complexes in cells. In the small complex (SC), which consists of CycT1 and Cdk9, the kinase is active and phosphorylates the C-terminal domain (CTD) of RNA Pol II, leading to increased processivity. In the large comlex (LC), which consists of the HEXIM1 protein and 7SK snRNA in addition to P-TEFb, the kinase is inactive. Tat can displace HEXIM1 from 7SK snRNA and recruit the now active P-TEFb to the HIV LTR. To date, the NMR structures of only the Tat arginine-rich motif (ARM) peptide-TAR RNA and argininamide-TAR binary complexes have been solved. Studies of Tat in the HARC Center aim to determine the structures of Tat-P-TEFb-TAR complexes, as well as those of a minimal Tat-CycT1 chimera bound to TAR and of the LC. These structures will be compared to those of Tat- TAR complexes to ascertain similarities and differences in the binding surfaces and bound conformations and in order to better understand the postulated roles of induced fit in Tat function. Rev mediates the nuclear to cytoplasmic export of unspliced and partially spliced HIV mRNAs. This activity involves the binding of approximately 6-10 Rev subunits to the Rev Response Element (RRE) RNA located in an intronic region of the unspliced transcripts. Rev then interacts with the host protein Crm1 (Exportin 1), forming a complex that is further stabilized by the binding of the GTP-bound form of Ran to Crm1. This entire RNA-protein complex is then moved through the nuclear pore and released on the cytoplasmic side, providing unidirectional export of the viral mRNAs. To date, the NMR structure of only the Rev ARM peptide-RRE RNA binary complex and a crystal/EM-derived structural model of the large Crm1 protein alone have been solved. Recent advances in Rev protein biochemistry will facilitate structural studies of intact Rev and its oligomeric complexes. Our proposed studies aim to understand differences in the recognition of different sites on the RRE, the potential for interactions among Rev subunits in the oligomeric RRE complexes, and the structural basis for Crm1 recognition by Rev-RRE complexes.